1. Field of the Invention
The present invention relates to an ink jet recording head that records on a recording medium by discharging ink and, more particularly, to an improved ink filtering system for such a recording head.
2. Description of the Related Art
In a conventional ink jet recording apparatus, in particular an apparatus that uses a permanent type ink jet recording head, the obstruction of the nozzle of the head with dirt or dust can shorten the life of the apparatus. Hence, to prevent dirt from mixing in an ink path of such an apparatus, a filter is inserted into an ink circulation system.
FIG. 7 shows the ink circulation system of this type of ink jet recording apparatus.
When the ink in a subtank ink housing 421 runs low, the ink drops from an ink cartridge 420 by gravity in order to make up the loss. A gap is provided where the subtank 421 and the ink cartridge 420 are fitted together. The interior of the subtank 421 is in communication with atmosphere by means of the gap. When ink drops from the ink cartridge 420 to the subtank 421, a volume of air equal to the amount of ink that has dropped enters from the subtank 421 to the ink cartridge 420.
At this point, prior to the use of an ink jet recording head 410, a recovery operation is performed for evacuating air present in the ink path of the apparatus and for circulating ink in the liquid chamber 411 of the ink jet recording head 410, the latter because the viscosity of the ink may have increased so the ink differs from its original quality. The recovery operation is performed such that the ink in the subtank 421 is supplied to the liquid chamber 411 by means of a gear pump 431 and a lower tube 440, by rotating a motor 430 to operate the gear pump 431. The ink is returned to the subtank 421 again by way of an upper tube 441. Some of the ink supplied to the liquid chamber 411 during a recovery operation may ooze out of some of the head nozzles 412, and any such ink is absorbed by a liquid absorbing agent 413 disposed below the head nozzles 412. At this time, it is effective to blow air from the head nozzles 412 so as to make easy the oozing out of ink to be dropped. As a result, the ink whose viscosity is increased in the head nozzles 412 is discarded and any dirt or debris present on the surface of the head is washed away.
In order for the recovery operation to function reliably, the gear pump 431 feeds ink at a high pressure of a lift of about 5 m and at a flow rate of about 1 cc/sec. Hence, there is a possibility that dirt in the subtank 421 and abrasive particles from in the gear pump 431 are fed by pressure into the liquid chamber 411 of the ink jet recording head 410.
The ink fed to the ink jet recording head 410 during recording is supplied from the subtank 421 by the upper tube 441. A small amount of ink may also be supplied through the lower tube 440 because of leakage through the gear pump 431, even though it is stopped. The ink at this time is approximately at atmospheric pressure (a lift of approximately 0.1 m) and its flow rate is 0.01 cc/sec or smaller. Therefore, the probability is small that dirt entrained in the ink is fed by pressure into the liquid chamber 411 through a first filter 414 and a second filter 415 which are disposed at the junctions of the ink jet recording head 410 and the lower tube 440 and the upper tube 441, respectively.
However, since the size of the openings of the first filter 414 is equal to the size of the openings of the second filter 415, and the two opening sizes are smaller than the minimum diameter of the head nozzles 412, this prior art apparatus can still be subject to clogging of the nozzles, because the shape of the dirt is not necessarily in the form of particles, but there also may be dirt present in the form of strings or filaments. Thus, even if the size of the openings of the nozzles 412 is greater than the size of the openings of the first filter 414, the nozzles 412 could be obstructed if dirt in the form of filaments, or filaments tangled with dirt particles, enter the nozzle 412.